1. Field of the Invention
This invention relates broadly to self-propelled projectiles. More particularly, this invention relates to rockets powered by hybrid propellant systems.
2. State of the Art
Rocket boosters (motors) generally fall into three classes: solid propellant boosters in which a solid fuel element, or grain, undergoes combustion to produce thrust that propels the rocket, liquid propellant boosters that accomplish the same function with a liquid fuel material, and hybrid boosters, described below. Solid and liquid rocket boosters can produce relatively large amounts of thrust, but for a relatively short amount of time. In addition, solid and liquid rocket boosters are generally expensive to develop and produce due to the inherent dangers of the highly combustible solid fuels and the complexity of bipropellant liquid feed systems.
Hybrid rocket boosters are described in detail in co-owned U.S. Pat. No. 5,715,675 to Smith et al., which is hereby incorporated by reference herein in its entirety. They have been characterized as a cross between a solid propellant booster and a liquid propellant booster. Generally, hybrid boosters use a fluid reactant (an oxidizer) to burn a solid fuel element, although they may use a combustible liquid fuel and a solid reactant. The solid element is generally formed as a thick-walled tubular cylinder, defining a port of a combustion chamber along its length. The hybrid rocket propellant (fuel and reactant together) can be ignited by a pyrotechnic igniter, such as an electrically-generated spark. The fuel of a hybrid rocket is inert until mixed with the oxidizer in the presence of an igniter in the combustion chamber. As such, there is no danger of inadvertent and uncontrollable combustion.
Hybrid rockets are subject to an oscillation in thrust level during burn of the propellant called xe2x80x9ccombustion instabilityxe2x80x9d. Combustion instability can vary from severe, oscillating between zero and one hundred percent thrust, to a currently acceptable standard of less than ten percent, and more preferably less than five percent. The inventors hypothesize that the instability is caused by the flame front drifting back and forth along the length of the fuel grain. The drift may be caused by injected oxidizer actually xe2x80x98blowing outxe2x80x99 the flame. The result of flame front drift is a change in the oxidizer/fuel ratio and combustion efficiency, as well as corresponding thrust levels. The instability is most evident in high mass flux ratio (Gt) motors, with the mass flux ratio calculated as propellant lbsxc2x7sec/area of the port measured in square inches. In a typical well-designed hybrid motor, an unacceptable level of combustion instability occurs with a mass flux ratio of 0.5 or more. However, a higher mass flux ratio is desired, as increasing the mass flux ratio permits the diameter of the port within the solid fuel to be reduced and the web thickness (thickness of the solid fuel wall) to be increased, thereby significantly increasing the volumetric loading of a motor, as well as burn time and total impulse.
It is therefore an object of the invention to provide a hybrid rocket motor which burns with high stability.
It is also an object of the invention to provide a hybrid rocket motor having a higher mass flux ratio.
It is another object of the invention to provide a hybrid rocket motor which increases volumetric loading, burn time, and total impulse.
In accord with these objects, which will be discussed in detail below, a rocket is provided which includes a hybrid motor, a casing about the hybrid motor, an aft nose cone, and a rear nozzle. The hybrid motor includes a storage tank which stores fluid reactant (oxidizer), a combustion chamber, a solid fuel grain defining a central port within the combustion chamber, and an injector adapted to inject the oxidizer into the combustion chamber. According to the invention, a flame holder is provided at a head end of the combustion chamber and maintains a flame adjacent the injector. The flame holder stabilizes the flame front and prevents the flame front from drifting along the fuel grain, which the inventors believe to be a cause of combustion instability, thereby reducing or eliminating combustion instability.
According to one embodiment, the flame holder includes a high-temperature casing defining a cavity at the head of the combustion chamber, and a solid propellant within the cavity around or near the injector. The propellant may be generally cylindrical within a cylindrical casing, such that the flame plume of the burning propellant is substantially perpendicular to the oxidizer flow, or may be provided in an annulus within the casing, such that the flame plume from the burning propellant is substantially parallel to the flow of the oxidizer. The solid propellant is preferably ignited substantially simultaneously with the ignition of the hybrid motor. The burning of the solid propellant prevents the flame which results from combustion of the fluid oxidizer and solid fuel in the hybrid motor from drifting, and thereby stabilizes the flame front for the hybrid motor.
According to another embodiment, the flame holder is a precombustion chamber supplied with propellant from separate fuel and oxidizer sources. The propellant can be in the form of gas or liquid and injected substantially tangentially into the head end of the hybrid motor adjacent the oxidizer injector to form a propellant swirl. As the hybrid motor oxidizer is injected into the swirl, it is heated and gasified, and assumes a swirling motion which increases the oxidizer path length and thereby increases the dwell time of the oxidizer. The increased dwell time increases combustion efficiency. The precombustion chamber can be held in an xe2x80x9cidlexe2x80x9d state (i.e., when no hybrid motor oxidizer is injected through the injector), allowing the hybrid motor to have multiple restarts without multiple pyrotechnic igniters. Additionally, the precombustion chamber fuel and oxidizer may be adjusted during hybrid motor burn as needed based on hybrid motor oxidizer flow rates.
According to a further embodiment of the invention, the injector is extended into the combustion chamber to form a toroidal precombustion chamber therebehind which has a controlled exit area (annular nozzle) adjacent a face of the injector. Solid fuel may be provided in the chamber or liquid fuel may be injected therein. In either case, oxidizer is injected into the precombustion chamber, either from a separate source or from a tap on the main oxidizer line. The oxidizer and fuel mix and travel in a swirling motion and generate heat sufficient to function as a flame holder. The heated flow is ejected from the exit area into the combustion chamber. The precombustion chamber heat generation may be controlled or interrupted by control of the flow of oxidizer into the precombustion chamber.
The additional heat and energy added to the head end of the combustion chamber vaporizes the oxidizer as it is injected into the combustion chamber, thereby maximizing surface area of the oxidizer, and reducing reaction time of the fuel-oxidizer propellants. This operates to ensure that a flame head does not drift and is stabilized at the injector.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.